If you are not a very experienced rider, or don't have enough money for good trainer than I'd go with dead quiet horse around 10-12 years old. You still will have LOTS of things to do with it (you can try barrels, jumping, name it), but you won't worry about possibility to be bucked off on trail of something spooks the horse.
If you have some experience (or at least a strong wish :) ), money, and time(!), you can get something little greener and put training on him/her.
It's really a matter of preference. I got 2 greenies and think I should of get just one and one dead broke, but.... Lol!